1. Field of the Art
The invention relates to catalysts for olefin polymerization. The catalysts of the invention are capable of providing high and prolonged catalytic activity for olefin polymerization and making it easy to control the molecular weight of the resulting polymers.
2. Prior Art
It has recently been suggested that polymerization of an .alpha.-olefin having 3 or more carbon atoms can be carried out in the presence of a solid catalyst comprising titanium, magnesium and halogen as essential elements to produce polymer with high stereoregularity. The suggested methods require, in addition to the solid catalyst and organoaluminum compound, a certain electron donor (external donor) compound during the polymerization reaction to increase the stereoregularity of the polymer product (for example, Japanese Patent Application Laid-Open Specifications No. 151,691/1977, No. 21,093/1978, No. 127,408/1980, No. 63,310/1982). Such use of an electron donor compound as a third component, however, makes the catalyst system complicated and makes it difficult to control polymerization, namely product properties in contrast to the advantages of an increase in the stereoregularity. Furthermore, the use of such electron donor generally shortens the catalyst activity.
In view of the above, the inventors developed a new catalyst system in which stereoregularity of the polymer product can be regulated without use of a certain electron donor compound (U.S. patent application Ser. No. 07/014,835 filed Feb. 2, 1987 and now Pat. No. 4,780,443.
On the other hand, control of molecular weight in a polymer product (molecular weight, distribution of molecular weight, etc.) is also an important element. Since the above mentioned electron donor compound as known as a third component does not control molecular weight sufficiently, new catalysts have been desired which make the molecular weight control possible without decreasing both the stereoregularity of a polymer and catalystic activity.